1. Field of the Invention
This invention relates to a pick-up head for Digital Versatile Disc (DVD), and more particularly, to a two-wavelength laser pick-up head which can be used for accessing DVD, Compact Disc (CD), CD-Recordable (CD-R) and CD-Erasable (CD-E) systems.
2. Description of the Related Art
The DVD pick-up head typically includes two focal points of different focal lengths. There are five different kinds of DVD pick-up heads described hereinafter. (1) FIG. 1, is a schematic diagram of a conventional DVD pick-up head of U.S. Pat. No. 4,399,529. As shown, this DVD pick-up head is a two-laser DVD pick-up head. One laser light of a single wavelength is used for writing. Another laser light is used for reading. Its disadvantage is that it is not applicable to the DVD systems. (2) The SONY DVD pick-up head is composed of two laser diodes and two pick-up heads. One is a DVD pick-up head, and another is a CD pick-up head. The disadvantage of the SONY DVD pick-up head is that it includes two objective lens. (3) The PANASONIC DVD pick-up head is composed of a laser diode and an objective lens. The disadvantage of the PANASONIC DVD pick-up head is that it includes an HOE objective lens which makes the efficiency of light lower. (4) The TOSHIBA DVD pick-up head is composed of a laser diode and two objective lens. (5) The SANYO DVD pick-up head is composed of a laser diode and an objective lens. Its disadvantage is the complicated design of the SANYO DVD pick-up head system.